Active continental margins and the active flanks of island arcs lie in the forearc regions of arc-trench systems generated by plate consumption. Arc-trench systems are initiated by contractional activation of previously rifted continental margins, by reversal of subduction polarity following arc collisions, and as island arcs within oceanic regions. The varied configurations of shelved, sloped, terraced, and ridged forearcs arise partly from differences in initial geologic setting, but mainly from differences in structural evolution during subduction. Forearc terranes enlarge during subduction through linked tectonic and sedimentary accretion of deformed ocean-floor sediments and igneous oceanic crust, uplifted trench-floor and trench-slope sediments, and the depositional fills of subsiding forearc basins. Trench inner slopes typically are underlain by growing subduction complexes composed of imbricate underthrust packets of oceanbasin, trench-floor, and trench-slope sediments in thrust sheets, isoclines, and melanges. The structure of subduction complexes is governed by the thickness and nature of oceanic layers rafted into the subduction zone, variable thicknesses of trench and slope sediments, and the rate and obliquity of plate convergence. Forearc basins between the magmatic arc and the trench axis include (a) intramassif basins lying within and resting upon basement terranes of the arc massif, (b) residual basins resting upon oceanic or transitional crust trapped between the arc massif and the site of initial subduction, (c) accretionary basins resting upon accreted elements of the growing subduction complex, and (d) composite basins resting upon more than one of the foregoing basement types. Strata deposited in forearc basins are typically immature clastic sediments composed of unstable clasts derived from rapid erosion of volcanic mountains or uplands of plutonic and metamorphic rocks within the arc massif. In equatorial regions reef carbonate associations are also common. Facies patterns of turbidites, shelfal sequences, and fluvio-deltaic complexes within forearc basins are governed by the elevation of the basin thresholds, the rate of sediment delivery, and the rate of subsidence of the substratum. Petroleum prospects in forearc regions typically are limited by complicated structures within the subduction complex, scarcity of good reservoirs in the forearc basin, often immature source beds, and low geothermal gradients except within the arc massif where heat flux is commonly excessive.
Plate tectonics has made possible an integrated analysis of the structural development of active continental margins and the flanks of active island arcs. Both are associated with plate consumption that generates arc-trench systems. Characteristic features include deep trenches marking the sites of plate consumption, chains or belts of volcanoes standing parallel to the trenches, and inclined seismic zones angling downward from the vicinity of the trenches into the mantle beneath the volcanoes. The seismic zones mark the course of plate descent. The arc volcanism evidently is triggered by descent of the plates into the atmosphere1.
